Each year, approximately 25 percent of US teenagers sustain physical, psychological, or sexual abuse by dating partners. Many victims of teen dating violence (TDV) experience a host of devastating consequences, including acute and chronic mental and physical health problems, suicidality, delinquency, risky sexual behavior, substance abuse, and academic failure. Moreover, perpetrators of TDV are at increased risk for continuing intimate partner violence in adulthood. The prevalence of TDV, as well as the adverse mental, behavioral, and relationship health outcomes associated with it, underscores the need for effective TDV prevention programs. For maximum benefit, these prevention programs must be developmentally well-timed and promote healthy relationships among all teens, which is why school-based programs hold considerable promise. One evidence-based program, the Fourth R, integrates the promotion of healthy relationship skills and prevention of TDV into existing school curricula. Thus, we propose to conduct a school-based cluster randomized trial of the 7th-grade version of Fourth R with 30 ethnically diverse middle schools (i.e., clusters: 15 intervention schools; 15 control schools) in one of the nation's largest school districts. Students (N = 3,375) in these schools will be the unit of analysis and will be studied prospectively (annually for 3 years) to determine the impact of the program by comparing students in intervention schools with those in control schools. The primary aim of this study is to determine whether Fourth R reduces students' experiences with TDV, as indexed by less perpetration and victimization of physical, sexual, and psychological TDV. The secondary aims of this study are to determine whether Fourth R (1) improves students' emotional well-being and increases their acquisition and use of healthy relationship skills, as indexed by improved problem-solving, communication, and conflict resolution skills and (2) ameliorates the modifiable cognitive and behavioral correlates associated with the perpetration and victimization of TDV, as indexed by fewer attitudes justifying dating violence, decreased substance use, decreased risky sexual behavior, decreased fighting and bullying, increased school connectedness, and decreased psychological symptoms. Prespecified differences by gender and ethnicity will be examined as well as the acquisition and utilization of various program components. Although TDV is a pervasive problem, it is largely preventable. In the proposed study, we will use a rigorous methodological design to test a new and enhanced version of Fourth R, one of the very few promising TDV prevention programs. In addition to overcoming limitations of previous TDV prevention research (e.g., small sample sizes, limited follow-up times, and ethnic homogeneity), the proposed study will be the first to rigorously evaluate Fourth R in a US sample and the first to test the effectiveness of the new 7th-grade version of Fourth R. A cost-effective and relatively portable TDV prevention program that can be embedded in US middle schools has the potential to dramatically improve the health and well-being of adolescents across the nation.